Drilling fluid

ABSTRACT

Zinc oxide and zirconium oxide are used as weighting materials for water and oil based drilling fluids for horizontal wells.

BACKGROUND

The present invention relates to drilling fluids of the type employed inoil and gas wells. More particularly, the invention relates to weightingmaterials for water and oil based drilling fluids for horizontal wells.

In drilling geopressured horizontal wells, a drilling fluid with densitybetween 12-19 ppg is often required. Traditionally, barite (bariumsulfate) is used as weighting material for drilling fluid. Barite is notsuitable for horizontal wells because a barite-laden filtercake cannotbe dissolved. Undissolved barite particles can invade the productionscreen in the horizontal well and plug it causing cessation or reductionin flow of hydrocarbon.

The present invention in response to this need in the art provides asolution as described hereinafter.

SUMMARY

The primary purpose of the present invention is to alleviate problemswith weighting materials in drilling muds used in drilling geopressuredhorizontal wells. To this end, zinc oxide and zirconium oxide are usedas weighting agents in such drilling fluids, preferably in particlesizes of the order of 10 microns or a range of 4 to 200 microns.

PREFERRED EMBODIMENTS

Zinc oxide is a particularly suitable material for weighting because, ithas a high density (5.6 g/ml versus 4.5 for barite); It is soluble inacids (e.g. HCl); and its particle size can be designed so that it doesnot invade the formation. Acid solubility is particularly desirablebecause dissolved ZnO can be produced through a production screenwithout plugging it. A high density means less weighting material isneeded per unit mud volume to achieve a desired density. Tests wereperformed with commercially available ZnO particles. Surprisingly, itwas found that the particle size (of the order of 10 microns) is suchthat the ZnO particles do not invade the formation core with thefiltrate. Also it is not large enough to settle out of suspension. Otherweighting material such as manganese tetraoxide is so fine that itinvades the formation with the filtrate. Zirconium oxide possessessimilar properties as ZnO. It has a density of 5.7 and is soluble innitric acid and hot concentrated hydrochloric, hydrofluoric and sulfuricacids, making it useful in the invention.

As above observed, these materials are acid soluble. Hence filtercakeformed from them can be dissolved and produced through the productionscreen without plugging it. Also they are fine enough to be easilysuspended in the oil or water base mud, but not too fine to invade theformation with the filtrate. The very high acid solubility of ZnO makesit particularly suitable as weighting material. This problem is uniqueto horizontal wells because they are often completed as open holeswithout a casing. A production screen is placed in the open hole toretain sand and yet allow hydrocarbon to be produced.

Solubility of ZnO in HCl at elevated temperatures was measured. At 150°F., solubility of ZnO in 15% w/v HCl is 60 lb/bbl and 98 lb/bbl in 25%w/v HCl. Return permeability experiments with ZnO weighted waterbase mudshowed that the ZnO mud did not damage the formation and that the ZnOfiltercake can be dissolved by HCl. Experiments also showed that anaqueous NaBr mud weighted with ZnO has acceptable rheology for drilling.

As above-mentioned, particle size is extremely important to theinvention. U.S. Pat. No. 3,622,513 to George Miller, patented Nov. 23,1961, incorporated hereinto by reference thereto, discloses the use ofzinc oxide in an oil base mud as a weighting agent. However, Miller usesvery small particle sizes of less than 3 microns. This would not besuitable for use with the present invention. The size of the particlesof this invention is such that it does not invade the formation corewith the filtrate and it is not large enough to settle out ofsuspension. Accordingly, the present invention is useful with a particlesize range of 4 to 200 microns, more preferably 6 to 100, and mostpreferably 8 to 50.

The present invention is suitable for use with all types of muds, bothwater base and oil base, as well as emulsion muds, including invertemulsion muds and synthetic muds. The invention is suitable for use withthe alcohol-in-oil drilling fluid described in U.S. Pat. No. 5,072,794to Hale et al, issued Dec. 17, 1991, which disclosure is incorporatedhereinto by reference thereto. The oil can be vegetable or mineral orcan be synthetic such as esters, isomerized olefins or polyalphaolefins. For water base muds the water can be fresh, brackish or brine.

We claim:
 1. A drilling fluid for use in geopressured horizontal wellscomprising a liquid carrier medium and a weighting agent of zinc oxideor zirconium oxide having a particle size range of 4 to 200 microns. 2.The drilling fluid of claim 1 wherein the particle size range is 6 to100 microns.
 3. The drilling fluid of claim 1 wherein the particle sizerange is 8 to 50 microns.
 4. The drilling fluid of claim 1 wherein theliquid carrier medium is water base.
 5. The drilling fluid of claim 1wherein the liquid carrier medium is oil base.